Power inverters are widely utilised to convert DC (direct current) power to AC (alternating current) power for applications requiring a time dependent power input. There are two generally well-known types of power inverter, namely current source inverters and voltage source inverters. Current source inverters have a predetermined load current, with the load impedance determining the output voltage, whereas voltage source inverters specify the load voltage, with the drawn current being dictated by the load.
In an example of a basic power inverter, a DC power source is connected to a transformer through the centre tap of the primary winding. A switch is used to rapidly alter the path of current flowing back to the DC power source, such that the current flows back to the DC source via two alternate paths, firstly through one end of the primary winding, and secondly through the other end of the primary winding. This alternation of the direction of current in the primary winding produces an alternating current in a secondary circuit of the transformer.
This basic principle of operation can be extended to output three-phase currents, which are commonly used for variable-frequency drive applications, for example as a power input for an electrical induction motor. A two-level, three phase voltage source inverter typically comprises three sets of two switches (a top switch and a bottom switch), with each set of two switches effectively functioning to produce a phase of the three-phase current that is to be produced.
A common fault in an inverter such as that described above is a so-called “open switch” fault, whereby a fault in the system causes a switch to remain open, thereby preventing the desired form of three-phase current being produced. This can prove to be costly in terms of machine down-time, increased operating costs, and further incurred costs associated with the repair of the open switch fault. It is therefore desirable, where possible, to prevent such open switch faults from occurring, or to quickly diagnose faults such that they may quickly be rectified.
Controlled power rectifiers operate in a similar manner to convert alternating current (AC power) to direct current (DC power), and thereby suffer from similar open switch faults
There has now been devised an improved method of open switch fault detection in a two-level voltage source power converter which overcomes or substantially mitigates the aforementioned and/or other disadvantages associated with the prior art.